The present invention relates to a method for producing a cyclic lactic acid oligomer, and a cyclic lactic acid oligomer produced by the production method.
A lactic acid oligomer having a cyclic structure is a useful compound which is used as a medicament such as a tumor cell growth inhibiting agent (Japanese Patent Application Laying-Open (Kokai) No. 3-193731) or an antineoplastic agent (Japanese Patent Application Laying-Open (Kokai) No. 9-227388), or an intermediate thereof.
The conventional method for producing such a lactic acid oligomer involves subjecting lactic acids to dehydration condensation by heating under an inactive atmosphere, and then separating and collecting an oligomer component from the obtained reaction products.
However, since it is difficult to produce a lactic acid oligomer selectively by this conventional method and that the lactic acid polymer obtained in the dehydration condensation process of lactic acids has a broad molecular weight distribution, containing high polymers, it is necessary to separate and collect a lactic acid oligomer by separation means such as chromatography.
The object of the present invention is to provide a novel method for effectively producing a cyclic lactic acid oligomer, and a cyclic lactic acid oligomer produced by the method.
As a result of concentrated research to achieve the aforementioned object, the present inventors have found that a cyclic lactic acid oligomer can be produced effectively by polymerization of lactides in the presence of a certain alkali metal compound, thereby providing the present invention.
Thus, according to the present invention, there is provided a method for producing a cyclic lactic acid oligomer represented by the following formula (1): 
wherein m represents an integer of 1 to 30,
wherein lactides are polymerized in the presence of an alkali metal compound represented by the following formula (2):
Rxe2x80x94Yxe2x80x94Mexe2x80x83xe2x80x83(2)
xe2x80x83wherein R represents an aliphatic group, aromatid group, xe2x80x94Si(R10)(R11)(R12), xe2x80x94CH(R20)CONR21R22 or xe2x80x94CH(R30)COOR31, wherein each of R10, R11 and R12 independently represents an aliphatic or aromatic group, R20 represents an aliphatic group, each of R21 and R22 independently represents a hydrogen atom, aliphatic group or aromatic group, R30 represents an aliphatic group, and R31 represents a hydrogen atom, aliphatic group or aromatic group;
Y represents xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94 or xe2x80x94NR40xe2x80x94, wherein R40 represents a hydrogen atom, aliphatic group or aromatic group; and
Me represents an alkali metal.
Preferably, the alkali metal compound is a compound of formula (2) wherein R represents an alkyl group having 1 to 12 carbon atoms, aryl group having 6 to 30 carbon atoms, xe2x80x94Si(R10)(R11)(R12), xe2x80x94CH(R20)CONR21R22 or xe2x80x94CH(R30)COOR31, wherein each of R10, R11 and R12 independently represents an aliphatic or aromatic group, R20 represents an aliphatic group, each of R21 and R22 independently represents a hydrogen atom, aliphatic group or aromatic group, R30 represents an aliphatic group, and R31 represents a hydrogen atom, aliphatic group or aromatic group.
Preferably, the alkali metal compound is a compound of formula (2) wherein Y is xe2x80x94Oxe2x80x94 or xe2x80x94Sxe2x80x94.
Preferably, the alkali metal compound is a compound of formula (2) wherein Me is lithium.
Preferably, in formula (1), m is an integer of 1 to 21.
According to one embodiment of the present invention, as the alkali metal compound, there is used any of: a compound of formula (2) wherein R is an aliphatic group having 4 or more carbon atoms; a compound of formula (2) wherein R is an aromatic group and Y is xe2x80x94Sxe2x80x94; or a compound of formula (2) wherein R is xe2x80x94CH(R20)CONR21R22 wherein R20represents an aliphatic group and each of R21 and R22 independently represents a hydrogen atom, aliphatic group or aromatic group. In the case of using such alkali metal compounds, cyclic lactic acid oligomer is selectively produced substantially free of chain lactic acid oligomer.
According to another aspect of the present invention, there is provided a cyclic lactic acid oligomer, which is produced by the aforementioned method for producing a cyclic lactic acid oligomer according to the present invention. Preferably, there is provided the cyclic lactic acid oligomer which is substantially free of chain lactic acid oligomer.